BUG-509: create data tree SPI package

This patch introduces an SPI package, which contains the metadata tree
node abstraction and reference implementation. This will substitute the
StoreMetadataNode class.

Major improvement is the isolation of lifecycle from the rest of the
implementation and major reduction in memory usage, as leaf nodes no
longer need to lug a HashMap around.

Change-Id: I89215672006b1ed6064feaf186c94c05550d299e
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: create data tree SPI package"